ORIGINAL: downunder
With the ST it sounds like you've stripped the thread in the brass nut.
So should only have to find a new carb thread
With the GMS, I gather you mean that with no prop fitted the crankshaft turns easily (at least while the exhaust port is open)
No, it does not flip over even with the prop removed. When I rock the crankshaft back and forth I can see the piston pumping up and down. Will not do a full revolution one way, it stops dead like something is blocking it.
If this is the case then tightening the prop is forcing the prop driver back too far on the tapered collet and it's hard up against the crankcase.
Not the case, as stated above does not turn with prop on or off.
This may be because (if the engine is new) the crankshaft isn't seated in the bearings fully forward or it's been in a crash that's pushed the crankshaft back.
Don't know what you are talking about with 'seated in the bearings' but its brand new, never in a crash and never even run yet.
Another possibility is that the prop driver/collet is too loose a fit and seats too far back.
Again no knowledge of what talking about.
BTW, this locking against the crankcase is fairly common on the ST's because their prop driver is cast, not machined from bar stock, and a crash or overtightening the prop can crack them.
That's exactly what it is doing. Locking against the crank case. But again, never been in a crash or ever run. And happens with or without prop as mentioned above.
SO . . . what could be causing it as explained?